The present invention generally relates to a sample mounting fixture for preparing samples for microscopic observation and more particularly, relates to an observable sample mounting fixture that can be used to mount a sample to a substrate while allowing the mounting and curing process be inspected through a window provided in the mounting fixture.
In the study of electronic materials and processes for fabricating such materials into electronic devices, a thin specimen is frequently required for analysis and for process validation. For instance, thin specimens are frequently used in the analysis of semiconductor structures by a transmission electron microscopy (TEM) method. TEM is one of the more popular methods used in analyzing the microscopic structures of semiconductor devices. The advantages achieved by a TEM method over that of a canning electron microscopy (SEM) method are higher magnification and simpler specimen preparation since no staining is required, even though a more three dimensional image can be obtained by the SEM method.
In preparing thin specimens of semiconductor structures for a TEM investigation. various polishing and milling process are involved so that specimens having thicknesses less than 1 xcexcm can be obtained. As device dimensions are continuously being reduced to the sub-half-micron level, the use of thin specimens for study by the TEM method becomes more important In general, when a thin specimen is prepared for a TEM study, various mechanical polishing methods are first used to bring the dimension of the specimen down to its approximate dimension. A final sample preparation process is then accomplished in a method called ion milling. The ion milling method is frequently conducted by a forced ion beam (FIB) technique. In the FIB technique, focused ion beams are used to either locally deposit or remove materials.
Before an integrated circuit specimen can be polished or milled, the surface of interest on the specimen must be protected. For instance, when a specimen is prepared for transmission electron microscopic (TEM) observation, a typical procedure used is to mount a protective glass slide on the top surface of the specimen. In order to mount the glass slide to the specimen, a layer of an epoxy adhesive is used. For observing the specimen in a transmission mode, it is preferred that the epoxy adhesive should be substantially clear or transparent.
In mounting a protective glass slide to the surface of an IC specimen, problem in bonding frequently occurs which leads to air bubble inclusions in the adhesive layer. The air bubble inclusions can be caused by a variety of factors, i.e., any residual cleaning solvent such as acetone that was left on the surface of the specimen may lead to air bubbles when the solvent evaporates during the adhesive curing process, inadequate mounting procedure when the glass slide is placed on the IC specimen or a defective adhesive material which produces air bubbles during a subsequent high temperature curing process. When air bubble inclusions occur in the specimen prepared, the specimen is rendered useless for subsequent microscopic examination due to optical distortion caused by the air bubbles. The problem becomes more serious when there is only one or very few specimens available for observation such that the risk of losing even one specimen is not acceptable. Any air bubble inclusions in the specimen therefore lead to the scrap of the specimen. This is because when air bubbles are observed after the adhesive curing process, the protective glass slide can no longer be removed without destroying the specimen surface that is to be examined. A reliable specimen preparation method is therefore a critical step in the quality assurance of IC chip fabrication process.
A conventional mounting method for an IC specimen 10 to a protective glass slide 12 is shown in FIGS. 1Axcx9c1C. In this conventional mounting method, an IC specimen 10 and a protective glass slide 12 are first provided, as shown in FIG. 1A. A substantially clear adhesive layer 20 is then coated on the top surface of the IC specimen 10 for mounting the protective glass slide 12 thereto. The sandwiched structure 22 is then placed between two teflon plates 14, 16 and clamped under pressure for curing in an oven. An oven curing temperature of at least 100xc2x0 C. can be used for a time period of about 15 minutes. This is shown in FIG. 1B. After the curing process is completed, the sandwiched structure 22 is removed first from the oven and then from the teflon plates 14, 16 as shown in FIG. 1C. Air bubble inclusions 18 observed in the adhesive layer 20 after the high temperature curing process render any TEM examination of the specimen impossible. It is further impossible to remove the glass slide 12 and the adhesive layer 20 without substantial damage to the surface of the IC specimen 10.
It is therefore an object of the present invention to provide a sample mounting fixture that can be used to mount an IC specimen to a protective glass slide that does not have the drawbacks or shortcomings of the conventional mounting fixtures.
It is another object of the present invention to provide an observable sample mounting fixture for mounting an IC specimen to a protective glass slide in an adhesive mounting process wherein the process can be observed in real time.
It is a further object of the present invention to provide an observable sample mounting fixture for mounting an IC specimen to a protective glass slide wherein an early detection of bubble formation in the adhesive layer is possible.
It is another further object of the present invention to provide an observable sample mounting fixture for mounting an IC specimen to a protective glass slide by a clear adhesive wherein air bubble formation can be detected in an early stage of the mounting process such that the glass slide can be removed and the mounting process can be repeated.
It is still another object of the present invention to provide an observable sample mounting fixture for mounting an IC specimen to a protective glass slide wherein the fixture is provided with an observation window such that an early detection of air bubble formation in the specimen surface can be visually made.
It is yet another object of the present invention to provide an observable sample mounting fixture for mounting an IC specimen to a protective glass slide wherein the mounting process can be monitored in real time by placing the fixture in an optical microscope.
It is still another further object of the present invention to provide an inspectable sample holder for mounting a sample to a substrate by using a sleeve bearing in the sample holder such that a vertically exerted pressure can be applied to the sample and the substrate.
It is yet another further object of the present invention to provide an inspectable sample holder for mounting a sample to a transparent substrate by providing an observation window in the sample holder such that the condition of the sample/transparent substrate can be monitored throughout the mounting process in an optical microscope.
In accordance with the present invention, an observable sample mounting fixture for mounting an IC specimen to a protective glass slide which can be continuously monitored during the mounting process is provided.
In a preferred embodiment, an observable sample mounting fixture is provided which includes a base portion and a top portion integrally connected together providing a cavity thereinbetween for receiving a sample, the base portion is provided with a window for observing the sample when positioned on the base portion, the top portion is provided with a recess therein for receiving a compression means, the recess further includes elongated slot openings through side walls of the top portion for receiving a handle of the compression means, the compression means has a shaft, a sleeve bearing for receiving the shaft in an upright position through a center aperture, a coil spring mounted on the shaft for pressing a compression foot formed on a bottom end of the shaft against a top surface of the sample when the sample is positioned on the base portion, and the coil spring has an upper end pressing against a lower surface of the sleeve bearing when the bearing is fixedly mounted in the recess in the top portion of the fixture such that the compression foot exerts a vertically downward force on the sample positioned on the base portion by the action of the coil spring.
In the observable sample mounting fixture, the window in the base portion may be formed of an aperture and a transparent plate covering the aperture. The handle may be integrally formed with the shaft of the compression means for gripping by human fingers when the handle protrudes through the elongated small openings to compress the coil spring and to lift the compression foot from the top surface of the sample. The base portion and the top portion may have a cross-sectional area of a rectangle, or may have a cross-sectional area of at least 2 cmxc3x973 cm. The sample may be a sample for transmission electron microscopy, or maybe an integrated circuit chip mounted to a glass slide by a transparent adhesive. The mounting fixture may be fabricated of a metal, or of aluminum. The compression foot may be formed of teflon.
In another preferred embodiment, an inspectable sample holder for mounting a sample to a substrate is provided which includes a holder body that has a bottom plate equipped with a see-through window, the bottom plate has a flat top surface for receiving the sample and the substrate thereon, a compression shaft equipped with a compression foot on a bottom of the shaft, a sleeve bearing frictionally engaging a recess in the holder body for vertically guiding the compression shaft through a center aperture in the bearing, and a coil spring acting between the sleeve bearing and the compression foot for vertically pressing the sample and the substrate together against the bottom plate of the holder body.
In the inspectable sample holder for mounting a sample to a substrate, the sample may be an integrated circuit chip and the substrate may be a glass slide. The sample and the substrate may be mounted together by a transparent adhesive. The compression shaft may further include a handle portion integrally formed therewith for gripping by human fingers for relieving a compressive force placed on the sample and substrate by the coil spring. The handle portion protrudes through elongated small openings in the holder body for gripping by human fingers. The holder body may be fabricated of a metal and dimensioned such that it fits inside a specimen stage of an optical microscope. The compression foot may be formed without a sharp point. The see-through window allows an observation of air bubble formation in a transparent adhesive layer between the sample and a substantially transparent substrate. A substantially transparent adhesive such as epoxy or polyurethane may be used to mount the sample to the substrate.